1. Field of the Invention
The invention relates generally to the field of seismic data acquisition and processing. More specifically, the invention relates to methods for processing seismic signals to attenuate the effects of certain types of noise in the seismic signals.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the Earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in marine environments, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the subsurface Earth formations until it reaches an acoustic impedance boundary in the formations. Acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are typically referred to as “bed boundaries.” At a bed boundary, some of the seismic energy is reflected back toward the Earth's surface. The reflected energy may be detected by one or more of the seismic sensors deployed on the surface. Seismic signal processing known in the art has as one of a number of objectives the determination of the depths and geographic locations of bed boundaries below the earth's surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the bed boundaries and back to the sensors at the surface.
Seismic surveying is performed in the ocean and other bodies of water (“marine seismic surveying”) to determine the structure of Earth formations below the water bottom. Marine seismic surveying systems known in the art include a seismic survey vessel which tows one or more seismic energy sources, and the same or a different survey vessel which tows one or more “streamers.” Streamers are arrays of seismic receivers or sensors disposed along a cable that is towed by the vessel. Typically, a seismic vessel will tow a plurality of such streamers' arranged to be separated by a selected lateral distance from each other, in a pattern selected to enable relatively complete determination of subsurface geologic structures in three dimensions. It is also known in the art to place cables having seismic sensors (“ocean bottom cables”) along the water bottom, and actuate a seismic energy source in the water. Typically, the seismic energy source will be towed by a vessel just as in streamer-type surveying.
At the bed boundaries, as previously explained, some of the energy from the source is reflected and ultimately detected by the seismic sensors. In addition to reflected seismic energy both coherent noise and incoherent noise may be present in the detected seismic energy. The presence of noise in the energy detected by the seismic sensors reduces the signal to noise ratio (“SNR”) of the seismic signals of interest. One objective of seismologists is, therefore, to seek methods of reducing the effects of noise on the signals detected by the sensors without appreciably reducing the true seismic signal component of the detected signals.
Prior art methods which have been used to reduce the effects of noise and acquire a higher quality representation of a particular subsurface structure include using multiple actuations of the seismic source (multiple “firings” or “shots”) to record a plurality of sensor measurements from substantially the same subsurface structure, and then summing or “stacking” such measurements to enhance signal strength while substantially reducing the effects of random or incoherent noise.
U.S. Pat. No. 5,818,795 which is assigned to an affiliate of the assignee of the present invention, and which provides a detailed summary of prior art methods and systems addressing the problem of noise suppression in seismic signals, discloses a method of reducing the effect of “burst” noise in seismic signal recordings without eliminating seismic signals of interest.
U.S. Pat. No. 5,761,152, which is assigned to an affiliate of the assignee of the present invention, describes a method and system for marine seismic surveying. The method disclosed in the '152 patent includes increasing the fold (number of recorded reflections from a same reflector), and as a result the signal-to-noise ratio of coherent seismic signals, without incurring the problems of drag, entanglement, complicated deck handling associated with increased streamer length, increased number of streamers, and increased distance between streamers. Source and streamer “offsets”, and time of firing of lead and trailing vessel sources in a time delay sequence are optimized to increase the fold while avoiding substantial influence by the seismic signals resulting from the source of one vessel on the seismic signals resulting from the source of the other vessel.
U.S. Pat. No. 6,751,999 issued to Fookes et al., which is assigned to an affiliate of the assignee of the present invention describes a method for attenuating noise from marine seismic signals caused by a noise in the water. The method includes determining an arrival time of a noise event at each of a plurality of seismic sensors, estimating a position of the noise source from the arrival times, and attenuating the noise event from the signals detected by the seismic sensors.
The foregoing description is not meant to be an exhaustive explanation of the types of noise and the methods for reducing the effects thereof in seismic signals. There are two types of noise, however, that continue to present a need for improved processing methods. One of these types of noise is generated by vessels or rigs in the vicinity of the seismic vessel (here termed “continuous ship noise”). The other type of noise is reflected impulsive noise which originates as a result of actuation of the seismic energy source and subsequent reflection of the seismic energy traveling laterally through the water to a reflector in the water, and then back to the sensors on the array (here termed “back scattered noise”). There continues to be a need for improved methods for attenuating back scattered noise and continuous ship noise in marine seismic data.